(CBS/AP) Morning-after pills were set to hit drug store shelves this month as the FDA was preparing to lift an age limit on the Plan B One-Step emergency contraceptive, allowing it to be purchased over-the-counter.

But Health and Human Services Secretary Kathleen Sebelius stepped in Wednesday and put the kibosh on that plan.

"It is common knowledge that there are significant cognitive and behavioral differences between older adolescent girls and the youngest girls of reproductive age," Sebelius said in a statement. "I do not believe enough data were presented to support the application to make Plan B One-Step available over-the-counter for all girls of reproductive age."

Plan B instead will remain behind the pharmacy counter, available without a prescription only for those 17 and older who show an ID proving their age.

Sebelius nixed the proposal at the eleventh hour because girls as young as 11 are physically capable of having children, and Plan B's maker, Teva Pharmaceuticals, didn't prove that younger girls could properly understand how to use this product without adult guidance.

Conservative groups lauded the decision.

"Take the politics out of it and it's a decision that reflects the concerns that many parents in America have," said Wendy Wright, an evangelical activist who has led opposition to Plan B.

Some experts disagreed with Sebelius' move.

"This decision is stunning," Dr. Susan Wood of George Washington University, who served as the FDA's top women's health official until resigning in 2005 to protest delays in deciding Plan B's fate, told Reuters. "I had come to believe that the FDA would be allowed to make decisions based on science and the public's health. Sadly, once again, FDA has been over-ruled and not allowed to do its job."

"I don't think 11-year-olds go into Rite Aid and buy anything," said Dr. Cora Breuner of the American Academy of Pediatrics. "We want it to be available to both girls and boys who have made a serious error in having unprotected sex and should be able to respond to that kind of lack of judgment in a way that is timely as opposed to having to suffer permanent consequences."

FDA Commissioner Dr. Margaret Hamburg stood by her agency, saying drug-safety experts had carefully considered the question of young girls for over a decade before she had agreed that Plan B's age limit should be lifted.

"There is adequate and reasonable, well-supported and science-based evidence that Plan B One-Step is safe and effective and should be approved for nonprescription use for all females of child-bearing potential," Hamburg wrote.

Plan B One-Step is a single pill that contains a higher dose of the
female progestin hormone that is found in regular birth control pills.
Taking it within 72 hours of sex can cut the chances of pregnancy by up
to 89 percent, but works best within 24 hours. There are two other
emergency contraception pills, a generic version similar to Plan B named
Next Choice and a prescription-only pill named ella. If a woman already
is pregnant, the pill has no effect.